1. Field of the Invention
The present invention relates generally to rate and power control in a communication system supporting multiple bit rates and power levels, and more specifically to rate and power control for determining and controlling the rate and power of a transmitter communicating with a receiver in a communication system. Even more specifically, the present invention relates to a rate and power control algorithm that may be used in a wireless communication network.
2. Discussion of the Related Art
In many communication systems, it is desirable to maximize system throughput or capacity. Throughput is a function of the signal-to-interference ratio (SIR) at a receiver and the modulation scheme used at a transmitter communicating with the receiver and may be defined as the number of bits that can be transmitted successfully to the receiver within each transmitted symbol. The SIR at a given receiver in a communication network is a function of the transmit power of all other users in the network or system. The transmit powers of unwanted users may cause interference (which may be referred to as co-channel interference) and can potentially reduce the SIR for the desired user or receiver. Co-channel interference is one of the main impairments that degrade performance in a wireless link.
One method to reduce the effect of co-channel interference is to employ power control techniques. One technique is known as the SIR-balancing approach in which a fixed target SIR must be met for signaling received at all terminals in the communication system or network. For example, in a TDMA cellular voice network, a fixed target SIR of 12 dB must be met for each user. Then, the transmit powers for each user in the system are constantly increased if the measured SIR of the signaling is too low (in comparison to the target SIR) for any one user while the transmit powers are decreased if the measured SIR of the signaling is too high in comparison to the fixed target SIR. Thus, this approach attempts to improve the weakest link within the communication system.
However, in the SIR balancing approach, the target SIR is fixed and is the same for all users in the system. This is due to the fact that there are no different quality of service (QoS) requirements in the system and that all links in the system are to be treated equally. For example, such systems only transmit voice or only transmit data traffic, each of which have a separate quality of service, i.e., a separate fixed target SIR to be met for each type of traffic. Such an SIR-balancing approach would not work effectively in a wireless multimedia communication system that supports multiple types of traffic (e.g., voice, data, video) simultaneously having multiple QoS requirements.
Furthermore, conventional SIR-balancing approaches have the potential to become unstable. A power change by an individual terminal in the system to maintain the fixed target SIR will in turn affect the interference seen by all other terminals and create some degree of positive feedback (instability) between the individual power control processes. Namely, when a terminal raises its transmit power to meet the fixed target SIR, the raise of power, in turn results in the increase of interference with other terminals. As a remedy, other terminals have to raise their own transmit power accordingly. This may lead to instability in the system in that all the terminals will raise their power until they all hit the maximum power limit. Since the SIR is a ratio, the terminals will eventually hit the upper limit of transmit power, without achieving the fixed target SIR.